The invention relates to a clamping device for attaching buoyancy bodies to a riser at offshore petroleum installations.
To reduce the weight of flexible risers in water; buoyancy modules are attached along the tube. Such buoyancy modules are manufactured of a buoyancy material, e.g. epoxy-resin, microglass and larger hollow plastic spheres. Such buoyancy modules typically have a specific weight of 0.35-0.45 and are dimensioned for depths of 500-2000 meters. The size is normally adapted to provide a net buoyancy of 500-2000 kg.
To attach buoyancy modules, clamping devices are coupled to the riser. The buoyancy module is then attached to the clamping device on the riser. The buoyancy is then transferred to the riser via the clamping device. With safety factor considered, such a clamping device should be dimensioned to take an axial load of 5-10 tons.
Such clamp should satisfy several, partly contradicting criterias:                It should be mechanically stable over a lifespan of 20-25 years        It should have a certain elasticity to compensate for expansion/contraction of the riser (due to changes in the internal pressure/temperature of the riser)        It should be applicable to risers with relatively large tolerances in the outer diameter (typically +/−5%)        The materials should not be influenced by hydrostatic pressure.        
This combination of demands is difficult to satisfy.
The coupling between the clamp and the riser is dependant on the surface friction of the contact surface between the clamp and the riser. Due to the structure and the material composition of the riser, a frictional coefficient of only 0.07 between the clamp and the riser is allowed. This requires a relatively high clamping force on the riser to achieve sufficient capacity.
The clamping device is typically manufactured with three-four segments and are clamped with a strap. This strap should be sufficiently flexible to allow for the changes in diameter occurring. The clamping force of the strap may typically be 20-80 tons, depending on the calculated force of the clamp.
Known clamping devices are mainly manufactured of polymeric material. A problem with such material is that the combination of relatively high exploitation of the material and relatively high temperatures will involve plasticity flow. Such flow involves dimensional changes reducing the holding capacity. To solve this problem, the clamping device can be made larger to reduce the tension, but this is undesirable.
From UK patent specification 2288205 (CRP Group 1995) a clamping device with two or more arched gripping elements with pressure elastic surface of contact or rigid gripping elements covered with a pressure elastic layer, is known. It is proposed to manufacture this clamping device of a foam material to provide buoyancy without additional bodies. To fasten this clamping device to the riser, a strap of metal is utilized.
The disadvantage of such a clamping device is the flow and deformation of the polymeric materials due to high tension in combination with high temperature.
From UK patent specification 2286649 (Balmoral 1995) a riser clamp is known, in which two internal substantially semicircular elements with series of annular ribs attached are forced against the riser with a strap of metal or a composite. The buoyancy elements are attached to the ribs.
A disadvantage of such clamping device, are the grooves in the contacting face of the clamp, to increase the desired holding force. Such holding means are not acceptable due to the risk of damaging the riser. Polymer material has been used in the clamping elements, with the risk of flow and deformation.
WO-Publication 2004090348 (Balmoral Group 2004) teaches the use of a shell around the riser and applying tension with a strap, the shell carrying ribs for attaching buoyancy bodies.
A disadvantage when using clamps of polymeric material, is the plasticity flow.